The Sensible Fuel System

Part II--From tank to injector, reliable service begins with attention to detail

The first article in our four-part series on diesel engines focused on fuel’s role in the combustion process (see “The Ghost in Your Machine,” June 2001), but fuel does more than just burn. Diesel fuel is slippery stuff, and that natural slipperiness, or lubricity, is one of the secrets to a diesel engine’s legendary longevity. Components immersed in diesel fuel are self-lubricating and, therefore, long lasting.

This high lubricity is particularly important in the fuel-delivery system, where the tolerances–the specified clearances between moving parts–can be infinitesimal. These minute tolerances call for clean, pure fuel–no water, air, or solid contaminants.

In the most common installation, fuel courses through the following sequence of components: tank, primary filter, lift pump, secondary filter, injection pump, injector. A small amount of fuel used to lubricate the injectors circulates through the injectors and usually returns to the tank via return fuel lines. The goal of all these components is
to deliver exactly the right amount of clean fuel from the tank to the engine at precisely the right moment.


One Tough Tank
The fuel tank’s design, construction, and installation deserve careful thought. Overlooking small details can lead to big headaches. (See “Tanks for the Memories,” October 2000.) The American Boat & Yacht Council’s (ABYC) Standards and Recommended Practices for Small Craft dedicates an entire section to the minimum acceptable standards for fuel systems. Anyone considering installing a tank or fuel system for the first time should review these guidelines and consult a professional.

Fuel tanks can be built of any of several materials: aluminum, plastic, Monel, or fiberglass/epoxy, among others. Because it can be easily cut, drilled, and welded, aluminum is the most popular fuel-tank material, although polyethylene is gaining a steady following. Aluminum resists corrosion by forming a protective film, and properly installed aluminum tanks will resist corrosion well.

The ABYC specifies that aluminum fuel tanks be made of alloys of grade 5052, 5083, or 5086. The minimum allowable thickness is no less than .090 inch, but larger tanks should be thicker–.125 to .250 inch–to meet structural requirements and provide a measure of security.


Constant moisture inhibits aluminum’s ability to combat corrosion. Any material that can absorb or hold water (including something as seemingly benign as a piece of paper) shouldn’t be allowed to stay in contact with an aluminum tank. Carpeting and plywood are strictly forbidden. A good solution is to mount the tank on a material that won’t absorb water, such as Delrin or King StarBoard.

Other important details in good aluminum tanks include removable fuel pickup tubes (no filters or screens) that are no closer than 5/8 inch from the bottom of the tank; baffles, if the tank is larger than 20 gallons; an inspection port in every baffled chamber; return fittings connected to drop tubes (removable tubes that extend from the top to the bottom of the tank); and female-thread fittings that are flush with the tank. All fittings or ports should be on the top of the tank; otherwise, one failed joint or fitting can lead to an empty tank and a fuel-soaked bilge.

Inspect tanks seasonally for corrosion and keep their insides as clean as possible. Remove one of the inspection covers once each season and scrape the tank’s bottom. If your scraper comes up black and gooey, it’s time for a thorough cleaning. A clogged filter is usually the first clue that the inside of your tank needs cleaning.


First-Rate Hoses
Fuel hose is rated by number and letter according to its purpose. While the ABYC only recommends type B1 fuel hose for diesel fuel, you’re better off with the more durable type A1. Type A1 hose is rated for gasoline. It’s less permeable, resists heat better, and is less likely to kink or collapse under vacuum.

Even type A1 wears out over time. All Coast Guard-approved marine fuel hose must bear a stamp every 12 inches indicating the manufacture date. Check hoses for dry rot, cracks, or sponginess at least once a season.

Between the tank and the injection pump, you can use seamless copper tubing instead of hose, with some important caveats. It should have a minimum wall thickness of .032 inch and have the proper end fittings for pipe-to-hose transition. Never clamp hose directly over a smooth piece of tubing; use only appropriate flare nuts and pipe fittings. When using metal tubing anywhere between the tank and engine, don’t attach the tubing directly to the engine because vibration and work hardening can cause it to fracture. Make the final connection to the engine with a liberal length of flexible hose.


The lines between the injection pump and the injector, which are under very high pressure, should be steel, not copper or aluminum alloys. Periodically inspect all metal fuel lines for leaks, cracks, rust, or bulges.

Any flexible fuel hose must end either with a swaged sleeve or a threaded insert or be clamped with a stainless-steel nonperforated clamp over a matching barbed fitting. Beware of so-called “stainless-steel” clamps with mild-steel screws that will quickly corrode. Seasonally inspect clamps for corrosion and a snug fit. Even stainless-steel clamps are susceptible to crevice corrosion (see “A Closer Look at Stainless Steel,” March 2001), especially under the screw. Proper clamp size is important. The minimum width of the bands on fuel clamps is 1/2 inch, and the band length should match hose diameter. If they don’t, sharp tails from ill-fitting clamps can turn your engine room into a razor-wire obstacle course.

All other plumbing fittings must be approved for fuel use. This excludes most plastic and PVC, unless specified for diesel applications. Many plastic fittings will become brittle after long exposure to hydrocarbons, and this will lead to leaks, spills, or air intrusion. Metallic fuel fittings must be galvanically compatible with each other. For instance, copper-alloy plumbing fittings must never be screwed directly into aluminum fuel tanks or filter bodies. Instead, use stainless-steel plumbing fittings or, at the very least, stainless-steel bushings.

The fewer unions fuel lines have, the less likelihood there is that air or fuel leaks will develop. If you’re replacing lines, make every attempt to install an unbroken run between the tank and the primary filter, and then from the primary filter to the lift pump. Secure all fuel lines–metallic or flexible–to prevent chafe and damage from vibration. Any metal fastening clips, especially when used to secure copper lines, should have a rubber bushing to prevent chafe and galvanic corrosion. Stainless-steel clips are preferable to aluminum, since they’re less likely to set up galvanic reaction with metal lines.

When inspecting for leaks, pay close attention to both the supply and return plumbing lines. The return fuel line should travel directly back to the tank. Plumbing return lines into the secondary filter works well under normal circumstances, but if an air leak were to develop in the return plumbing, it would contaminate the supply and affect the engine’s performance. An air leak in a return line that is plumbed back to the tank won’t cause this problem.

Flirting with Filters
Primary Filter: Without a doubt, dirty fuel causes some of the most common problems in diesel engines. Ironically, these problems are also among the easiest to prevent. The first line of defense is good filtration, and lots of it. The primary filter–the first filter that fuel encounters as it flows from the engine–should be able to hold a great deal of coarse dirt and water without impeding fuel flow to the engine.

The ideal primary filter should incorporate the following features: a clear plastic bowl (equipped with a heat shield if it’s located in the engine room), a positive-closing brass drain valve with a plug to prevent spills, a filter element that can be replaced without letting air into the system, a vacuum-gauge attachment and water sensor, and filter elements that are easily and readily obtainable. The Racor 500MA series filter (not to be confused with the automotive version, the 500FG) meets all of these qualifications. This filter can handle up to 60 gallons per hour (including return fuel), which is several times more than most sail auxiliaries would use. However, the added filter surface area and other key attributes that aren’t found in smaller filters make it worth the expense.

Diesel fuel filters are typically rated in microns–millionths of a meter. The higher the number, the coarser the filter. Most primary-filter manufacturers recommend using 10- or 30-micron elements, which means they’ll catch anything bigger than that size. The idea is to catch the water and coarse dirt here and let the secondary filter take care of the finer grit.

Some cruisers prefer to use a finer primary filter to catch all but the finest particles before they reach the secondary
filter. This may work for some, but in my experience, it isn’t any more effective. With this system setup, filters typically don’t last as long, and the engine is more likely to shut down due to acute fuel starvation from a clogged primary filter.

Lift-Pump Filter: Often, the next filter fuel encounters after the primary filter is inside the lift pump, which draws fuel from the tank and through the primary filter and then delivers the fuel to the injection pump. Many electric lift pumps incorporate a cylindrical paper element. This should be renewed along with the primary and secondary filters. Some mechanical lift pumps have a screen beneath the domed cover. You can wash this screen with clean diesel and reuse it. Always use lint-free rags when cleaning the insides of filter bodies or fuel pumps. Even the smallest debris will cause an injection pump to malfunction.

Secondary Filter: The final line of defense is the secondary-filter element. Commonly located on the engine, this filter is usually either a spin-on filter or a replaceable-cartridge element. The filtration rating varies among manufacturers; it’s usually in the range of 2 to 7 microns.

It’s best to use only the original equipment manufacturer’s elements in primary filters. The fit is right, and the necessary O-rings will be supplied. However, some quality name-brand equivalents may be suitable for engines with spin-on or cartridge elements in the secondary filter.

When replacing any filter, take great care when removing the old O-ring. New O-rings can be difficult to install in some housings, especially when you’re working against gravity. Smear a very light coating of grease on the O-ring before you insert it into its groove. This will help to hold the O-ring in place while you reassemble the filter body. Square-section O-rings often twist, which will let air into the fuel lines. If necessary, use a mirror and flashlight to ensure this doesn’t happen.

Pumps with Oomph
Diesel engines usually have two fuel pumps: lift pumps and injection pumps. The lift pump, as previously mentioned, transfers fuel from the tank to the injection pump. The pressure side of these is usually fairly low, around 6 to 15 pounds per square inch (psi). The vacuum side is typically capable of lifting fuel 5 to 7 feet. This means that a deep keel tank combined with a slightly dirty filter element might overwhelm a lift pump.

Lift Pump: Lift pumps are either mechanical or electric. Mechanical pumps usually use a diaphragm and check-valve arrangement, which is actuated from an engine camshaft lobe. (See “Know Your Pumps Inside and Out,” April 2001.) Some of these pumps also use a manual pumping lever to aid in bleeding the air out of the fuel system. (See “Painless Bleeding,” page 83.) Electric pumps are often square, roughly 2 by 2 inches; others are 6- by 2-inch cylinders. These also are usually diaphragm pumps. Electric pumps make it easier to bleed air from the system, since all you have to do is switch the ignition key to the “on” or “preheat” position to activate the pump. A push-button switch that can activate the pump from inside the engine compartment makes it easier for one person alone to bleed the fuel lines.

Depending on the design of the lift pump, it may need replacement or rebuilding after 3,000 engine hours. Many manufacturers offer lift-pump rebuild kits for mechanical pumps, and a competent handyman equipped with a service manual and parts can rebuild the pump.

Injection Pump: The other major pump in the fuel system is the injection pump. This mechanical marvel is the heart of the fuel system. Injection pumps are by nature robust. They’re designed to work under extreme pressure, heat, and vibration, yet the machining finish inside is exact. The injection pump receives fuel at a few psi and increases it to over 1,000 psi while sending it to the right injector at the precise moment. This is called timing, and it must be correct to within thousandths of a second.

Auxiliary engines typically use two types of injection pumps: inline jerk pumps and distributor pumps. An inline pump is recognizable by the row of fuel lines usually lined up across the top of the pump. This piston-style pump incorporates a separate plunger and barrel for each engine cylinder. Each plunger must deliver exactly the same amount of fuel as its neighbors; otherwise, the engine will run unevenly. An inline pump usually incorporates a reservoir for oil that lubricates some of the internal components. Over time, this oil eventually becomes contaminated or thinned by diesel and will require replacement. Check your owner’s manual to determine how frequently it needs to be changed.

The other style of injection pump is the distributor type. This piston-style pump has a single high-pressure plunger and cylinder that supplies high-pressure fuel, which is then distributed to each cylinder via a rotating-valve assembly. This pump is common on smaller diesels and is usually identifiable by its cylindrical shape and the location of the fuel lines, which exit horizontally from one end rather than the top of the pump body. Other than external cleaning and oil changes, injection-pump service should be entrusted only to experienced professionals in pristine workshops. Fortunately, injection pumps are built to endure. Depending on the model of pump and how well the fuel system has been maintained, inspection and servicing may be advisable after 3,000 hours.

Injector-Care Problems
The importance of clean, properly operating injectors can’t be overemphasized. These are precisely machined devices that ultimately “inject” the fine mist of fuel into each cylinder. A malfunctioning injector can lead to any number of problems: poor fuel economy, rough running, smoking, hard starting, overheating, and, potentially, seizure. The variety of injectors found on today’s marine diesels all have one thing in common: Maintenance is a must, and it should be carried out at specified intervals.

Like the other components in the fuel system, the internal parts of the injector are constantly bathed in lubricating diesel fuel. However, the injector tip, or nozzle, lives a hellish existence within the cylinder. It’s exposed to extremely high temperatures, over 1,000 F, and to air pressures in excess of 500 psi. It also must cope with the byproducts of combustion: soot, water, partially burned hydrocarbons, and acids. It’s no wonder that injectors usually require service more often than any other part of the diesel fuel system except filters. Manufacturer’s recommended service intervals vary. At the very least, injectors should be removed, cleaned, and inspected by an experienced diesel mechanic every 1,000 hours. Often they may need to be rebuilt or replaced (the replacement of inexpensive injectors is more economical than rebuilding). Cruisers who’ll be far from maintenance facilities should carry a complete set of spare injectors.

Diesel engines are reliable as long as they’re properly maintained. You can keep yours ticking over by carrying out regular maintenance and by keeping the fuel supply clean.

Steve D’Antonio is a contributing editor at Cruising World.